Wasn’t there a very similar question in Labrynth? Two talking doors, one of which is the right way, one alway lies, one always tells the truth…damn it…now I’m going to have to go and rent Labrynth…

Well, that uses up your one question, but how does it get you any closer to the water?

If the person answers with, “I’d say it was over there” you’re still no further ahead than when you asked - maybe they told the truth, maybe they lied.

The basic question is: Where is the water?

There are two possible responses. The truthteller will tell the truth, the liar will lie.

If asked, “If I were to ask you where to find water, what would you say”:

a) The truthteller will tell the truth about what she would say: that equals the truth.

b) The liar will lie about lying: that equals the trutn.

Or you could just say…”hey, let me buy you a drink.” Even a liar would perk up and lead you to the nearest bar at the offer of a free pint.

STageleft, it doesn’t matter which one you ask, that’t the point: by framing the question in a question, you compel the liar to tell you the truth = eg., to lie about his lie.

Cottonwood has it.

“If you were one of the others, which way would you point to show me to the water?”

Then take the opposite path.

As B notes, the key is to frame the question within a question and include one of the possible answers. You have to be dealing with a binary choice (and in the problem as presented will generally be faced with people who answer only “Yes” or “No”).

First frame the decision; for example, “X is true”, as in “I should take the left-hand fork in the road”, “I should open the right-hand door”, “I should go to that well if I wish to obtain water”.

If you ask, “Is X true?” and X is in fact true, the truth-teller will answer “Yes” and the liar will answer “No”, and vice versa if X is in fact false.

But, if you ask, “If I ask you whether X is true, what will you answer?” (or “…will you answer Yes?), then:

1) The truth-teller must say “Yes” if his answer to “Is X true?” would be “Yes” and “No” otherwise - his answer is the same to the direct or indirect question.

2) The liar must say “Yes” if his answer to “Is X true?” would be “No” (ie. if X is true) and “No” otherwise.

In both cases, “Yes” is the answer elicited if X is true and “No” if X is false, so you know whether X is true.